1. Technical Field
Applicant""s invention relates generally to coils for use in cast resin or other resin bonded structures such as transformer coils, and more particularly, to a method of forming cooling channels within the coil.
2. Background Art
A transformer generally consists of a laminated, ferromagnetic core, high voltage windings, and low voltage windings. The windings of dry type transformers with primary voltages over 600 volts have generally been constructed using one of three types of techniques: conventional dry, resin encapsulated, or solid cast. The conventional dry method uses some form of vacuum impregnation with a solventless type varnish on a completed assembly consisting of the core and the coils or individual primary and secondary coils. The resin-encapsulated method encapsulates a winding with a resin with or without a vacuum but does not use a mold to contain the resin during the curing process. This method does not insure complete impregnation of the windings with the resin and therefore the turn to turn insulation and layer insulation must provide the isolation for the voltage rating without consideration of the dielectric rating of the resin. The solid cast method utilizes a mold around the coil, which is the principal difference between it and the resin-encapsulated method. The windings are placed in the mold and impregnated and/or encapsulated with a resin under a vacuum, which is then allowed to cure before the mold is removed. Since all of the resin or other process material is retained during the curing process, there is a greater likelihood that the windings will be free of voids, unlike the resin encapsulated method whereby air can reenter the windings as the resin drains away before and during curing. Cooling channels can be formed as part of the mold. One type of such a transformer is manufactured by Square D Company under the trademark of Power-Cast transformers. Since the resin used in coating solid cast coils results in a greater solid bond between adjacent conductors than is possible with resin encapsulated coils, solid cast coils exhibit better short circuit strength of the windings. An added benefit is that by having greater mass, there is a longer thermal time constant with the solid cast type coils and there is better protection against short term overloads
Various methods are available whereby cooling channels are formed within the channels. U.S. Pat. No 4,129,938 discloses one such method. Duct forming molds, wrapped in cloth, are inserted during the winding of the coil at predetermined locations. They are extracted by force after the resin has cured. Removing these molds can be very time consuming. The duct molds are usually machined tapered metallic segments, either steel or aluminum. The machining is necessary to create large cooling ducts that will match the contour of the coil. These coils are generally cylindrical or at least have major cylindrical components. Since there are a variety of coils, having varying dimensions and sizes, it becomes necessary to have many different duct molds ready available to accommodate the different sizes. An alternative have been to use aluminum or steel bar stock, coated with a suitable mold release compound, for relatively small cooling ducts. After the resin has cured, the bars are extracted under force, which again, can be time consuming. These bars are usually straight segments with no curvature. They must be relatively narrow or the space factor of the coil is adversely affected.
It would be preferable to find a lower cost method for forming cooling ducts that eliminates the need for many different duct molds to accommodate all the variations in coil size. In addition, it would be preferable that the duct molds be easier to extract from the cured coil, thus reducing the overall cycle time for manufacturing the complete coil.
Accordingly, the principal object of the present invention is to provide an improved method for forming cooling ducts in a cast resin or other resin bonded structure such as a transformer coil which overcomes the above mentioned disadvantages.
Another objective of the invention is to provide a universal duct mold for forming the cooling ducts in a variety of different sized coils.
In one embodiment of the invention, a uniform cross section thermoplastic tooling element or segment is used to create cooling ducts in a cast resin transformer coil. These segments are wound into the coil between predetermined layers or radial sections, oriented parallel to the coil axis. They extend through the height of the coil. After the coil is cast, impregnated, or saturated with resin, and the resin is cured, the segments are withdrawn. The removal of a segment results in a continuous cooling duct.
The thermoplastic segments may be machined from sheet stock. For relatively small coils they may be injection molded or cast. Solid segments are preferred over hollow extrusions because they are more durable. The preferred manufacturing method is to machine them from sheet or bar stock.
Thin walled extrusions with suitable reinforcement could also be used. They are somewhat flexible, which will cause the radius of curvature of the segment to conform to the shape of the coil due to the tension from the conductor and fiberglass reinforcing material during the coil winding process. Longitudinal reinforcing bars or tubes can be used to prevent the thin walled plastic extrusion from collapsing from tension in the winding. This will have the result of reducing the number of different segment designs required to support production of a variety of different diameter coils. Elliptical or similar shaped coils with conforming cooling ducts are possible from the same segment design. Unreinforced thick walled extrusions are less durable because they tend to collapse after repeated uses. Any hollow extrusion should be fitted with a flexible plug in the lower end to prevent the entry of liquid resin.
The thermoplastic material is selected based upon its ability to resist reacting or bonding with the coil bonding resin. It also will have a heat distortion temperature high enough to resist the heat associated with the resin curing process. Further, the thermoplastic material will have a coefficient of expansion greater than the coil bonding resin system to facilitate removal of the segment after curing.
Many variations of the segment design are possible, including using snap together elements, joined edge-to-edge to create varying length cooling ducts.
Other features and advantages of the invention will be apparent from the following specification taken in conjunction with the accompanying drawings in which there is shown a preferred embodiment of the invention. Reference is made to the claims for interpreting the full scope of the invention that is not necessarily represented by such embodiment.